Control system for clothes dryer



May 31, 1966 J. B. KRlPKE 3,253,347

CONTROL SYSTEM FOR CLOTHES DRYER Original Filed Nov. 27, 1959 5Sheets-Sheet 1 w as INVENTOR.

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CONTROL SYSTEM FOR CLOTHES DRYER Original Filed Nov. 27. 1959 5Sheets-Sheet 5 4? w w #76; 1 w

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ATTORNEY United States Patent 3,253,347 CONTROL SYSTEM FOR CLOTHES DRYERJoseph B. Kripke, Birmingham, Mich, assignor to General MotorsCorporation, Detroit, Mich, a corporation of Delaware Originalapplications Nov. 27, 1959, Ser. No. 855,812, now Patent No. 3,110,005,dated Nov. 5, 1963, and Nov. 24, 1961, Ser. No. 154,713, now Patent No.3,169,838, dated Feb. 16, 1965. Divided and this application Apr. 27,1964, Ser. No. 362,872

. 6 Qlaims. (Ci. 34-45) This invention relates to a domestic applianceand more particularly to an improved control system for a clothes dryer;and is a division of my copending applications S.N. 855,812, filedNovember 27, 1959, now Patent 3,110,005 issued November 5, 1963 and SN.154,713, filed November 24, 1961, now Patent 3,169,838 issued February16, 1965.

The clothes drying art has long sought a method whereod whereby thedrying cycle may be terminated at the correct end point automatically.Past efforts toward a dryness control have been directed primarily tosensing sudden changes in dry bulb temperature as an indication ofclothes dryness. However, relatively few attempts have successfully tiedthe termination of a clothes drying operation to the moisture content ofthe fabric. One of the difficulties with this latter approach is in thedevelopment of a humidity sensor which is inexpensive to manuf-actureand which will be dependable over an extended life of operation. Adryness control to be completely effective, should be able to compensatefor different types of fabric, the weight of the fabric and the weightof the water in the load being dried. Accordingly, it is to the solutionof these and other problems that this invention is directed.

It is an object of this invention to provide a humidity sensor whichsenses the actual moisture content or absolute humidity of thesurrounding air.

Another object of this invention is the provision of a humidity sensorfor a variable temperature air stream, said sensor having a constanttemperature reference for stabilizing sensor operation.

It is also an object'of this invention to provide a humidity sensor fora clothes dryer which is cooled by a constant temperature reference tocompensate for the advance in dryer ambient temperatures withconsecutive drying cycles.

A further provision of this invention is the provision of a humiditysensor having a base reservoir portion and a moisture attractingportion, said base reservoir portion containing an hygroscopic saltwhich is given up gradually to said moisture attracting portion as areplenishment for salt leached from said attracting port-ion.

It is a further object [of this invention to sense the amount ofmoisture present in an air stream and to provide to a balanced circuitamplifier a signal as a function of the percent of moisture sensed.

Another object of this invention is the provision of a humidity sensorwhich depends upon the amount of moisture present to create a parallelresistance path be tween the adjacent turns of a wire coil.

It is also an object of this invention to terminate a fabric dryingcycle in accordance with fabric moisture content automatically.

It is also an object of this invention to terminate the drying cycle ofa fabric load when the load is at a predetermined degree of dryness.

A further object of this invention is the provision of a dryness controlwhich can compensate for type of fabric, weight of fabric and weight ofwater in the fabric load.

A still further object of this invention is the provision of a dryingcycle which is terminated automatically with the clothes at a suitabletemperature for comfortable handling.

Another object of this invention is the provision of a drying cycle withan automatic temperature responsive cool-off period at the end of adrying cycle.

A more specific object of this invention is the provision of a humiditysensing element which is comprised of a hollow support tube encased witha layer of porous material mixed with an hygroscopic salt, a sleeve ofabsorbent wicking and a helically wound coil of insulated highresistance wire, said coil having a non-insulated portion inelectricalcontact with said sleeve.

Further objects and advantages of the present invention will be apparentfrom the following description, reference being had to the accompanyingdrawings wherein preferred embodiments of the present invention areclearly shown.

In the drawings:

- FIGURE 1 is a sectional view of a clothes dryer suitable for use withthis invention;

FIGURE 2 is a sectional view of the humidity sensing device of thisinvention;

FIGURE 3 is a fragmentary sectional view taken along line 3-3 in FIGURE2;

FIGURE 4 is a fragmentary sectional view taken along line 4-4- in FIGURE3 to show the relationship of the insulated and non-insulated portionsof a single resistance wire convolution to the moisture attractingwicking of the sensing element;

FIGURE 5 is a schematic circuit diagram illustrating the electricaltheory of the humidity sensing arrangement of this invention;

FIGURE 6 is a schematic wiring diagram for controlling a clothes dryerwith the humidity sensing device of this invention;

FIGURE 7 is a diagrammatic view of a clothes dryer controlled by amodified version of the humidity sensing arrangement of this invention;and

FIGURE 8 is a fragmentary sectional view partly in I elevation of amodified mounting arrangement for the humidity controller of thisinvention.

In accordance with this invention and with reference to FIGURE 1, aclothes dryer 10 is shown comprised of a casing 12 having a rear wall14, a front wall 16 and a control housing 18. Disposed between the topwall 12 and bottom wall 13 of the casing is a vertical drum supportbulkhead 20 having a centrally located universal support bearing 22 inwhich a drum shaft 24 is rotatably mounted. The tumbling drum 26 issupported by and affixed to the shaft 24 at one end thereof and issupported at its other end on a port plate 28 carried by the front wall16 of the dryer cabinet. More particularly, the front wall 30 of thetumbling drum includes an axially directed outwardly turned cylindricalflange 32 which overlies an inturned flange 34 on the port plate 28.Sealing materialsuch as felt 38 may be interposed along with a pair ofbearing support blocks between the flanges 32 and 34 for rotatablysuper-ting the tumbling drum 26. The rear wall 40 of the tumbling drumis perforated as at 42 to place the interior of the tumbling drum incommunication with an annular chamber 44 adjacent the rear wall of thetumbling drum. The chamber 44 communicates by way of an opening 46 inthe bulkhead 20 with a heating chamber 48 in which a main heater 50 isdisposed. Both the cabinet rear wall 14 and the heating chamber 48 haveopenings as at 52 and 54, respectively, to permit a series flow of airbetween the atmosphere and the interior of the tumbling drum. As the airleaves the front of the tumbling drum, it communicates with a front duct56 by means of a passageway 58 interposed between the access door 60 andthe port plate 28. A removable lint screen 61 may be interposed in theair fiow path within the front duct 56. Beneath the tumbling drum 26 andwithin the dryer cabinet, a conventional blower 62 is disposed which hasan inlet 64 connected by means of a flexible conduit 66 to an outlet 68from the front duct 56. The blower outlet 70 communicates by means of asecond flexible conduit 72 with a cylindrical exhaust or humiditysensing element support housing 74 in which a humidity sensing assemblyshown generally at 76 is disposed. The assembly includes a humiditysensing element which is suspended by struts 77 in the housing 74. Theblower 62 and the tumbling drum 26 are both operated by a conventionalmotor (shown schematically in FIGURE 6)-the blower impeller 63 by meansof a shaft 78 and the tumbling drum 26 through a belt and pulley systemshown generally at 80.

The air flow system of the dryer which is suitable for use with thisinvention is designed as follows. With the blower 62 operating, dryingair will be drawn into the dryer cabinet through the opening 52 in therear wall 14 thereof. This air will enter the heating chamber 48 bymeans of the heating chamber inlet 54 and will pass over the heaters 50prior to entering the annular chamber 44 adjacent the rear wall 40 ofthe tumbling drum 26. The heated air is drawn equally through all of theperforations 42 in the drum wall 40 and will flow around and through theclothing being tumbled by the rotating drum 26. At this point themoisture in the damp clothes is vaporized and is entrained in the airflow passing through the tumbling drum. This moisture-laden air willthen be drawn through the passageway 58 and into the front duct 56 wherethe temperature thereof will be sensed by a thermostat or temperatureresponsive device 57, which may be adjustable in accordance with thedrying temperatures desired by the user. The thermostat 57, in turn,controls the primary heater 50. From the front duct 56 the air will befiltered through a conventional lint screen 61 prior to entering theduct or conduit 66 leading to the inlet 64 of the blower. On the outletside of the blower the moisture-laden air will be forced by way of theconduit 72 to the humidity sensing support housing 74 in which thehumidity sensor assembly 76 is disposed. At this point in the air flowsystem the absolute humidity or moisture content of the air will besensed and a signal indicative of such moisture content will be sentfrom the sensing device 76 to a dryer control shown generally at 84.When the air indicates a dry condition for the clothes within thetumbling drum 26, the heater 50 will be permanently deenergized and thedrying cycle terminated with a predetermined cool-ofi period duringwhich the fabrics reach a temperature suitable for handling.

Prior attempts to terminate a drying cycle by sensing humidity have metwith difliculties in developing a humidity sensing element which willoperate over extended periods with consistent operating characteristics.Further, the prior devices have been unable to recognize or distinguishbetween different fabrics or different size drying loads. Thisshortcoming stems primarily from the fact that the prior devices areunable to sense slight variations in moisture content carried by afastly moving air stream. The humidity sensing arrangement 76 of thisinvention overcomes these problems in a device which may be massproduced at extremely low cost and which has an unusually long anddependable life.

Reference may now be had to FIGURES 2, 3 and 4 for a completeunderstanding of the construction of this novel humidity sensing elementor sensor 100. The purpose of this element is to sense the amount ofmoisture present in an air stream passing thereover by offering achanging resistance indicative of the moisture present to a balancedcircuit amplifier. The functioning of the sensor depends upon the amountof moisture present to create a parallel resistance path betweenadjacent turns is comprised of a hollow tubular shaft or cylinder 102 ofaluminum or other suitable heat conducting material. The tube orcylinder 102 is encased by a layer of plaster of Paris .104 or otherporous plaster-like moldable material which has been mixed with asolution of an hygroscopic salt such as lithium chloride dissolved inwater. Since the plaster of Paris layer 104 is porous, this portion ofthe element 100 serves as a reservoir or retainer for excess hydroscopicsalt throughout the life of the element. Overlying the plaster of Parislayer 104 and concentric therewith is a sleeve 106 of asbestos wickingor other absorbent material. The wicking 106 lies in juxtaposition tothe plaster of Paris layer 104 so that it may receive the lithiumchloride or hygroscopic salt from its plaster of Paris reservoir.Although the plaster of Paris has been said to be mixed with lithiumchloride, it should be recognized that any other hygroscopic salt may beentrapped in the pores of the porous layer 104 for example, lithiumbromide. It is important only that an hygroscopic material be placed ina position of reserve adjacent the wicking material so that the wick maybe replenished with this hygroscopic salt throughout the life of thehumidity sensing element 100. In this way salt leached from the wickingis continually replaced from the porous reservoir.

The foregoing components of the sensing device, namely the wicking 106and the hydroscopic salt reservoir 104, provide the means for attractingand holding the moisture from the air being passed thereover. In orderfor the sensor 100 to be effective in controlling an electricalappliance, it is necessary that means he provided for interpreting thehumidity indicating signals from the sensor. The signals are in the formof a total resistance across the sensor 100 which varies in accordancewith the moisture entrapped by the sensor. To provide an interpretableresistance path in conjunction with the wicking material 106, it isnecessary that a coil 108 of very fine high resistance wire be wrappedaround the asbestos wicking sleeve 106. This wire may be of Nichrome.002 inch in diameter and may have a resistance of one hundredsixty-nine ohms per foot. As this resistance wire 108 is extremely fine,it is necessary that a support mandrel 110 be utilized as the carrierfor the resistance wire 108. Thus, the resistance wire is tightly,helically wound around the mandrel 110. After the resistance wire 108 issnugly wrapped about the mandrel 110, the mandrel 110 is then woundhelically about the asbestos sleeve wicking 106. It is important to notethat the mandrel 110v is nothing more than a carrier or mechanicalsupport for the wire 108 and performs no electrical function. The wire108 is electrically insulated from the mandrel 110 and the mandrel formsno portion of the electrical control circuitry of the sensing element100. Both the mandrel 110 and the resistance wire 108 are insulated orcoated by an enamel 112 on the mandrel and 114 on the'resistance wire108. Prior to winding the resistance wire wrapped mandrel about theasbestos sleeve 106, the insulation 114 is removed as at 116 to exposethe bare wire of the resistance conductor 108. Thus, after the mandrelis wrapped about the asbestos sleeve, the uninsulated portion 116 of theresistance wire 108 is embedded in the asbestos wicking 106 while aninsulated portion 118 extends above the asbestos wicking 106. In otherwords, the insulation is removed from the surface of the resistance wirecoil where the wire comes in contact with the asbestos wicking.Completing the humidity sensor 100 is a first terminal or spadeconnector 122 connected as by a bolt 124 to the cylindrical tubing 102.At the other end of the tube 102 is a second terminal or spade connector126 conected in similar fashion. The resistance Wire coil is providedwith a first terminal end 128 electrically connected to the spade orterminal 122 while the other terminal end 130 of the resistance coil iselectrically fastened to the terminal spade or connector 126. When thehumidity sensor 100 is placed in a control circuit, the sensor 100 maybe connected by means of the spades or connectors 122 and 126 into thecircuit to be humidity responsively controlled. The en tire sensor 100is light, compact and adapted for easy installation withinanyconventional appliance air conduit.

The electrical theory involved in connection with the humidity sensingelement 100 is shown schematically in FIGURE 5. The schematic isdepicted as though the resistance coil 108 were laid out flat on acontinuous layer of asbestos or absorbent wicking. Note that between theterminal ends 128 and 130 there are a plurality of coil segments orconvolutions 132. The convolutions of the coil 108 provide a fixedresistance path between the terminal ends 128 and 130. However, thebared or noninsulated portion of the convolutions 132, which extend intothe asbestos wicking 106, provide variable resistance paths 134 betweeneach convolution 132 of the resistance coil. As shown in the FIGURESschematic, each convolution 132 of the resistance coil 108 provides anincrement of resistance r The variable resistance path through theasbestos wicking 106 provides a variable resistance in accordance withthe moisture content of the asbestos wicking as indicated by rTherefore, the total resistance (r is equivalent to the resistance (r ofone convolution 132 of the wire coil 108 times the resistance (r of thevariable resistance to current flow path through the moisture-ladenasbestos wicking divided by the resistance (r through one convolution132 of the resistance wire 108 plus the resistance (r of the in formulaform variable resistance path through the asbestos wicking, or

This formula depicts both parallel paths, one of which is fixed througheach convolution of the resistance coil 108 and the other path of whichis variable through the variable moisture-laden asbestos wicking 106between the exposed adjacent uninsulated portions 116 of the coil 108.The main resistance R of the coil 108 alone is equal to N times r (R=N1- wherein N equals the number of convolutions or turns of theresistance coil 108 wrapped about the mandrel 110 between the terminalends 128, 130. On the other hand, the total leakage resistance (R3 isequivalent to the sum of all of the resistance paths between adjacentconvolutions through the moisture-laden asbestos wicking. Thus the totalresistance (R) including both main resistance and leakage resistance isequal to N times r (R=Nr To increase sensitivity of the humidity sensingelement 100 it is necessary only to increase the main resistance (R ofthe coil 108. The leakage resistance (R varies only with the moisturepresent in the asbestos wicking sleeve 106. Thus, we have a resistancepath disposed in an air stream which has parallel electricallyconducting paths. One of such paths has a fixed resistance and the otherof such paths has a variable resistance. When a current is passedthrough the coil 108 by connecting the sensor 100 into an electricalcircuit, such as for a dryer, a signal is given out or indicated by thesensor 100 which varies in accordance with the amount of'moistureabsorbed by the asbestos wicking 106. Such absorbed moisture providesthe variable flow paths between adjacent convolutions of the uninsulatedportions 116 of the resistance wire. The function of the asbestoswicking 106 is to absorb the moisture from the surrounding air toprovide a conductive path between the adjacent convolutions 132 of theresistance coil. As aforesaid, the resistance of the leakage pathbetween exposed portions of the coil 116 varies with this amount'ofmoisture in the asbestos wick. Since each of these moisture createdvariable resistance paths is in parallel with one convolution of theresistance coil 108, the result is a sensor whose overall resistancemoisture path, is variable.

With such a sensor design an element is provided which has a definiteresistance value for any given absolute humidity. A control can be madeto operate at a predetermined absolute humidity by turning the controlcircui-t to that resistance which correlates the desired humidity with adesired clothes dryness condition. Since a fast moving air streamcontaining moisture is reluctant to deposit such moisture on the surfaceof the asbestos wick, a material is needed which will attract themoisture. Thus, the porous plaster of Paris reservoir 104 may beimpregnated with an hygroscopic salt, such as lithium chloride, or othermaterial capable of attracting moisture. The amount of lithium chlorideused to impregnate the base 104 is selected so as not to over saturatethe asbestos wick 106. Note that the asbestos wick 106 is not saturatedwith the lithium chloride or hygroscopic salt directly. The salt isplaced in solution with the water which is used to mix the plaster ofParis which underlies the asbestos sleeve 106. The strength of solutionused is determined by empirical study in order to secure the correctamount of salt transmisison between the reservoir and the wicking.

Although the humidity sensor may be used in 'many applications where itis desired to control in accordance with absolute humidity, itsapplication has been found particularly effective in a clothes dryer.When so applied, a control circuit seen in FIGURE 6 is utilized tooperate the dryer of FIGURE 1 in response to the signals originatingwith the sensor 100. The overall humidity sensing arrangement 76includes the sensor 100, a starting bimetal or thermally responsiveswitch -operating on a fixed contact 142, and a cool-down bimetal orthermally responsive switch 144 operating on a fixed contact 146. Thesensor 100 and more particularly the resistance coil 108 is adapted tobe installed in a housing'or duct 74 through which air exhausts from thedryer 10. Both the cool-down switch 144 and the start bimetal switch 140are disposed in the same general area so that both the temperature andhumidity condition of the exhaust air may be sensed. Since the sensingcoil 108 produces or indicates signals in the neighborhood of fiftymicroamps, it is desirable to have in the dryer control 84 an amplifierincluding a solenoid 152 which is selectively energized by the signalsfrom the sensing coil 108. The circuitry of FIGURE 6 may include also adoor switch 154 for interrupting dryer operation whenever the door 60 isopened. A motor 156 is connected to the belt and pulley system 80 forrotating the tumbling drum 26 and to' the shaft 78 of the blower 62 forinducing an air flow through the tumbling drum. Disposed within thecircuit for heater 50 is the thermostat'57 which selectively interruptsenergization of the heater in accordance with a predeterminedtemperature selection amenable to the fabric being dried. A conventionalmotor speed switch 158 may be included to prevent operation of theheater should the motor fail to operate in driving the blower orrotating the tumbling drum. In accordance with this invention thecircuitry includes a manually closed master switch 160 which is adaptedto be opened by a bimetallic actuating member 162 which is flexed oractuated by a 'heater 164 to terminate automatically the drying cycle aswill be described more fully hereinafter.

The function of the start bimetal switch 140 acts to short out the coil108 of the sensor 100 to prevent the dryer from shutting down tooquickly, i.e. to sense falsely an indication of dryness before thefabric within the tumbling drum has started giving up its moisture tothe heated air. There is a period, generally below exhaust airtemperatures of approximately 116 F. while the air is being heated atthe beginning of the cycle, that substantially dry air would be sensedat the coil 108. This bimetal 140 serves to shunt out coil 108 duringthis period so that the sensor is in effect made inoperative untilmoisture is entrained in the air exhausting from the dryer.

The cool-down bimetal switch 144 functions at the end of the' dryingcycle and is adapted to close upon the contact 146 at temperatures belowapproximately 120 degrees. This provides the cool-down operation whichtends to terminate the drying cycle with a period of tumbling inrelatively cool air for wrinkle-free results. Once the sensing coil 103has indicated to the amplifier 150 that the clothes are dry, a cycle isestablished placing the cool-down switch 144 in the circuit as will beunderstood more fully hereinafter.

In operation a drying circuit provided with a sensor 100 of thisinvention is energized when the switch 160 is manually closed. Currentwill flow from L2 through the switch 160, a switch blade 170 of relay152, line 172, through the thermostat 57, the heater 50, the safetyswitch 158 (closed when the motor 156 is in operation) to L1. At thesame time, power will flow from L2 through the start switch 160, line174, line 176, the door switch 154, the motor 156 to the neutral side ofthe line N. Thus, the heater 50 and the motor 156 are in operation and adrying cycle is initiated. With the energization of the blower 62 bymeans of the motor 156, air will start flowing past the sensor 100 andmore particularly the resistance coil 108. This first relatively coolair will also he relatively dry and will falsely indicate a drycondition for the clothes. For this reason the bimetallic switch 140 isplaced in parallel with the resistance coil 108 and adapted to be closedbelow approximately 115 F. Of course, with the heater 50 energized, theair flowing past the sensor 100 will soon heat up and will start tovaporize the moisture from the clothes load being tumbled in the drum26. As the temperature rises above 115, the switch 140 will lift fromthe contact 142 and the sensing element 108 will be placed solely in thecircuit with the amplifier 150 as the means for amplifying andevaluating the humidity responsive signals produced. The amplifier 150is provided with a control knob 180 which acts through a potentiometerto balance the circuit within the amplifier in accordance with thedegree of dryness of the fabric desired. When the resistance balancedinto the amplifier 150 matches the resistance total across the coil 108,the solenoid 152 will be energized to move the relay switch blade 170 toa contact 182, thereby conditioning the cooldown circuit forenergization. At this time current will fiow from L2, start switch 160,the relay switch 170, contact 182, line 184, the bimetallic disconnectswitch heater 164 and line 186 to the cool-down bimetallic switch 144and from there to the other side of the line N. So long as temperatureswithin the exhaust housing 74 are above approximately 120 F., thebimetallic switch 144 will be opened and the heater 164 therebydeenergized. As the temperatures reduce below the 120 F. setting, thebimetal 144 will close, the cool-down circuit will be energized and theheater 164 will start warming the bimetallic actuating element 162. Inapproximately twenty-five to forty seconds after the bimetallic 162 isheated, the master switch 160 will be opened and the drying cycleterminated. During this cool-down period, the warm temperature of thedried clothes will be reduced to a suitable temperature for handling andthe relatively cool air passing over the tumbling clothes will aid inminimizing wrinkles. Any suitable amplifier 150 may be used in a circuitequipped with this invention. The amplifier could be transistorized orpower transistors might be considered to eliminate the relay in thecircuit.

Where the clothes dryer 10 is being used for one drying cycle afteranother without a cool off period between, it has been found desirableto alter the mounting arrangement for the sensor 100 shown in FIGURE 1.With repeat cycles the sensor tends to heat up and resists absorbingmoisture from the air at the start of the later cycles. For this reason,a modified mounting arrangement is shown in FIGURE 8, wherein the hollowcylinder 102 of the sensor 100 is connected to a conduit 200 extendingthrough a wall of the air flow system duct 66 leading to the inlet ofthe blower. The duct 66 may be replaced by the support housing 74 ofFIGURE 1the size, of course,

being adjusted to the distance between the blower inlet 64 and theoutlet 68 of the front duct 56. In any event struts 201 act to suspendthe sensor centrally in the air stream. Thus, the blower 62 serves todraw air from the outside atmosphere (dashed anrows) through the centeror inside of the sensor by way of conduit 200 while the moisture-ladenedair from the tumbling drum is drawn about the outside of the sensor(solid arrows) and in contact with the moisture absorbing asbestoswicking 106. With this arrangement, the dryer 10 may be used throughoutrepeat cycles without the sensor 100 changing in operatingcharacteristics. The relatively cool and stable temperature of theatmosphere provides a constant temperature reference for the sensor 100.Therefore, each drying cycle may be initiated with the sensor in aconstant temperature state.

Another embodiment whereby the sensor 100 of this invention may be usedto operate a modified clothes drying cycle is shown in FIGURE 7. In thisarrangement a dryer 210 is provided with a tumbling drum 212 connectingthrough a perforated rear wall 214 with a heating element 216. At theopposite end of the tumbling drum 212 the air flow is connected by meansof a front duct 218 with a blower 220, the outlet of which is connectedto the support housing 222 which may be substantially the same as 74described in connection with the foregoing or prefenred arrangement. Inthe FIGURE 7 circuitry, a heater 224 is inserted within the hollowsensor 100 and adapted to vary the rate at which moisture will beabsoxbed by the wicking 106 of the sensor. The hotter the sensor 100,the greater will be the resistance to taking moisture out of the dryerexhaust air. The output of the heater 224 is controlled by a variableresistance controller 226 which is positioned in accordance with apredetermined clothes dryness programmed by an operator through the knob228. This knob 228 adjusts a controller or amplifier 230 in accordancewith the signal received from the sensor 100 and in turn positions arheostat or variable resistance controller 232 in the circuit to theheater 216. In this way a constantly changing heat supply is providedfor the tumbling drum 212. Since clothes can stand more heat when wetthan when dry, high heat is provided at the start of each drying cycleand is diminished gradually as the clothes drying cycle progresses tothe selected dryness end point.

In operation the humidity sensing circuit of FIGURE 7 includes thehumidity sensor 100 (FIGURE 2) which is intended for use as the inputfurnishing element in a humidity. control circuit. It consists of thecylinder 102 supporting the absorbent material such as the asbestoswicking 106. Within the hollow cylinder a heating element 224 is mountedand the entire assembly is then positioned within an exhaust duct orchamber 222. As moist air passes through the duct 222, the porouswicking material will be penetrated by moisture. This moisture picked upfrom the clothes being dried acts to vary the resistance of the element100 in the control circuit, causing the primary heating element 216 tochange its heat input to the air entering the tumbling drum 212. Theamount of moisture in the passing air establishes the amount of moisturetrapped in the porous cylinder of wicking 106. As this moisture isdiminished or dries out, the resistance across the sensor 100 isincreased and this signal passed on to the controller 230. By means ofconventional relay devices in the controller 230 this signal isinterpreted and the variable resistance 232 is altered to reduce thepower supply to the primary heater 216. When the sleeve of wicking 106is completely dry, the resistance of the sensor 100 is at a maximum andthe primary drying action will be terminated with the deenergization ofthe heater 216.

In FIGURE 7 the output of the sensor heater 224 has been made adjustablethrough a variable resistance 226. Thus, the extent of the drying effectof the heater 224 on the moisture trapped in the porous wicking of thesensor can be controlled and set at specific levels depending on thenature of the material within the tumbling drum to be dried.

Another method whereby the heater 224 may produce an adjustable effecton the sensor is to make the penetration of the heater 224 within thecylinder 102 variable-the heat output of the heater 224 remaining fixed.In this way, the more that the heater 224 inserts within the tubularsupport element 102, the more reluctant will be the wicking to taking onthe moisture from the tumbling drum air passing thereover. That is, thesensor 100 will be heated and dry out in direct proportion to the amountof heater insertion.

By tying a humidity sensor to a controller for an infinitely variableprimary heating element for a dryer, it is possible to use the maximumtemperatures within the tumbling drum 212 which the fabrics canwithstand. Thus, as wet clothes are placed in the dryer, the maximumheat output of the heating element 216 may be used. As the clothes startto dry, this heat output is gradually reduced until the fabric iscompletely dry, at which point the output of the heating element 216will be reduced to zero and the drying cycle terminated. By tying theheat output to the dryness of the clothes, the duration of a dryingcycle may be reduced to the absolute minimum.

It should now be seen that an improved humidity sensing element has beendevised for use as a controller in a domestic dryer or for use in anyhumidity control arrangement. The design of the sensor whereby aquantity of hygroscopic salt is entrapped in a porous base material forgradual release to a moisture absorbing wicking is believed to embody ahumidity sensor design which will have exceptionally long life withconstant operating characteristics. A helically wound coil which isinsulated in an air stream but noninsulated in a wicking base, may beused to provide a variable resistance path which is indicative of theabsolute humidity of any fluid medium passing thereover. A signal soindicated may then be transposed through auxiliary amplifying equipmentto operate an appliance such as a domestic clothes dryer.

In addition a control cycle has also been devised whereby the humiditysensing element of this invention is used in conjunction withtemperature responsive devices to effect proper dryer operation duringthe warm up portion of a drying cycle and during the cool-down portionof the cycle, the latter to prevent wrinkles in the fabric being dried.A further concept of this invention is embodied in a drying systemwherein the output of the primary heater is diminished as the moistureis removed from the fabric. More particularly, as the moisture entrainedby a humidity sensor decreases, a signal is produced indicative of suchmoisture decrease and the output of the primary heater reduced inaccordance therewith. Such a system is particularly effective in savingon the consumption of electricity as well as in effecting a dryingcycle, the total duration of which is kept to a minimum.

While the embodiments of the present invention as herein disclosed,constitute preferred forms, it is to be understood that other formsmight be adopted.

What is claimed is as follows:

l. A control system for a damp fabric dryer having a casing defining adrying chamber and means for efiecting an air fiow through said chamber,said control system comprising, variable power supply means, heatingmeans connected to said variable power supply means and operable to heatsaid air flow to evaporate the dampness from said fabric to apredetermined dryness end point, humidity sensing means for sensingdampness in said air flow and having absorbent means in communication.with said air flow for absorbing moisture therefrom as a measure of thedampness thereof, said sensing means including electrical conductormeans responsive to the moisture absorbed by said absorbent means forcontrolling said variable power supply means for progressively reducingthe supply of power to said heating means in accordance with aprogressively decreasing dampness sensed in said air flow, and drynesscontrol means settable for predetermining the dryness end point of saidfabric and including means operable to regulate the ability of saidabsorbent means to absorb moisture from said air flow for varying thesensitivity of said sensing means to the dampness in said air flow,thereby to control the supply of power to said heating means throughsaid variable power supply means.

2. The control system of claim 1 wherein said means operable to regulatethe ability of said absorbent means to absorb moisture from said airflow includes a heating element in heat transfer relationship to theabsorbent means of said sensing means.

3. The control system of claim 1 wherein said means operable to regulatethe ability of said absorbent means to absorb moisture from said airflow includes a heating element in heat transfer relationship to theabsorbent mean-s of said sensing means and presettable means forcontrolling said heating element to regulate the heat transferred to theabsorbent means of said sensing means.

4. A control system for a damp fabric dryer having a casing defining adrying chamber and means for effecting an air flow through said chamber,said control system comprising, variable power supply means, firstheating means connected to said variable power supply means and operableto heat said air flow to evaporate the dampness from said fabric to apredetermined dryness end point, humidity sensing means for sensingdampness in said air flow and having a porous, absorbent wickingmaterial in communication with said air flow for absorbing moisturetherefrom as a measure of the dampness thereof, said sensing meansincluding electrical resistance means responsive to the moistureabsorbed by said wicking material for controlling said variable powersupply means for progressively reducing the supply of power to saidfirst heating means in accordance with a progressively decreasingdampness in said air flow as evidenced by the moisture in said wickingmaterial, and dryness control means settable for predetermining thedryness end point of said fabric and including a second heating meansoperable to heat said wicking material to regulate the ability thereofto absorb moisture from said air flow, said heating of said wickingmaterial varying the sensitivity of said sensing means to the dampnessin said air flow, thereby to control the supply of power to said heatingmeans through said variable power supply means.

5. The control system of claim 4 wherein said sensing means is definedby a cylinder wrapped on the outside thereof with said electricalresistance means and said wicking material, and said second heatingmeans is inside said cylinder.

6. A control system for a damp fabric dryer having a casing defining adrying chamber and means for effecting an air flow through said chamber,said control system comprising, variable power supply means, firstheating means connected to said variable power supply means and operableto heat said air flow to evaporate the dampness from said fabric to apredetermined dryness end point, humidity sensing means for sensingdampness in said air flow and having absorbent means in communicationwith said air flow for absorbing moisture therefrom as a measure of thedampness thereof, said sensing means including electrical resistancemeans responsive to the moisture absorbed by said absorbent means forcontrolling said variable power supply means for progressively reducingthe supply of power to said first heating means in accordance with aprogressively decreasing dampness in said air flow as evidenced by themoisture in said absorbent means,

and dryness control means settable for predeterrnining the dryness endpoint of said fabric and including a second heating means operable toheat said absorbent means to regulate the ability thereof to absorbmoisture from said air flow, said heating of said absorbent means vary-1 1 ing the sensitivity of said sensing means to the dampness in saidair flow, thereby to control the supply of power to said heating meansthrough said variable power supply means.

References Cited by the Examiner UNITED STATES PATENTS 2,166,799 7/1939Crago 236-44 2,320,474 9/ 1941 Ross 3448 X 2,403,630 7/ 1946 Blunk.

Root 236-51 Hughes 3445 Gray 34-45 X Thorsheim 34-45 X Sidar'is 34-45 XWILLIAM F. ODEA, Primary Examiner.

NORMAN YUDKOFF, Examiner.

D. A. TAMBURRO, Assistant Examiner.

1. A CONTROL SYSTEM FOR A DAMP FABRIC DRYER HAVING A CASING DEFINING ADRYING CHAMBER AND MEANS FOR EFFECTING AN AIR FLOW THROUGH SAID CHAMBER,SAID CONTROL SYSTEM COMPRISING, VARIABLE POWER SUPPLY MEANS, HEATINGMEANS CONNECTED TO SAID VARIABLE POWER SUPPLY MEANS AND OPERABLE TO HEATSAID AIR FLOW TO EVAPORATE HE DAMPNESS FROM SAID FABRIC TO APREDETERMINED DRYNESS END POINT, HUMIDITY SENSING MEANS FOR SENSINGDAMPNESS IN SAID AIR FLOW AND HAVING ABSORBENT MEANS IN COMMUNICATIONWITH SAID AIR FLOW FOR ABSORBING MOISTURE THEREFROM AS A MEASURE OF THEDAMPNESS THEREOF, SAID SENSING MEANS INCLUDING ELECTRICAL CONDUCTORMEANS RESPONSIVE TO THE MOISTURE ABSORBED BY SAID ABSORBENT MEANS FORCONTROLLING SAID VARIABLE POWER SUPPLY MEANS FOR PROGRESSIVELY REDUCINGTHE SUPPLY OF POWER OF SAID HEATING MEANS IN ACCORDANCE WITH APROGRESSIVELY DECREASING DAMPNESS SENSED IN SAID AIR FLOW, AND DRYNESSCONTROL MEANS SETTABLE FOR PREDETERMINING THE DRYNESS END POINT OF SAIDFARIC AND INCLUDING MEANS OPERABLE TO REGULATE THE ABILITY OF SAIDABSORBENT MEANS TO ABSORB MOISTURE FROM SAID AIR FLOW FOR VARYING THESENSITIVITY OF SENSING MEANS TO THE DAMPNESS IN SAID AIR FLOW, THEREBYTO CONTROL THE SUPPLY OF POWER TO SAID HEATING MEANS THROUGH SAIDVARIABLE POWER SUPPLY MEANS.